The invention pertains to self-expanding stents and particularly to methods of attaching axial filaments to the stent body to increase its radial expansion force.
Self-expanding stents such as braided or knitted stents for surgical implantation in body lumens (tubular vessels) are known for repairing or strengthening the vessels. A stent essentially is a hollow tube that supplements the body vessel. With respect to the medical condition of stenosis, in which a body lumen tends to collapse or otherwise close, the stent supports the wall of the vessel to prevent it from collapsing or closing. A blood vessel that is narrowed due to the build up of intra-vascular plaque is one example of a stenosis. With respect to the medical condition of aneurism, in which a body lumen is weakened and cannot properly withstand the internal pressure within the vessel and bulges out or ruptures, the stent serves essentially the opposite function in that it substitutes for or supplements a weakened portion of the vessel. Stents are known for insertion in blood vessels, bile ducts, colons, trachea, bronchi, esophagi, urethra, ureters, etc.
Many different types of stents are commercially available at this time. Most stents need to be radially constricted, i.e., reduced in diameter, so that they can be inserted into the body lumen. Once they are in situ, the stent can be radially expanded to the desired diameter. Stents are known that are fabricated from rigid, but deformable materials that, when bent by force, tend to retain the bent shape. Such stents may be inserted into the body lumen in an unstressed radially minimal shape while mounted over a deflated balloon. When the stent is in situ, the balloon is inflated in order to radially expand the stent, which will then retain the radially expanded shape after the balloon is deflated and removed.
Another type of stent is termed a self-expanding stent. Self-expanding stents can be compressed radially, but will expand to their original shape once the radially constricting force is removed. Some types of self-expanding stent are formed from materials that are superelastic or have shape memory characteristics. Such stents are commonly made of Nitinol, a biocompatible alloy that, depending on its chemical composition and thermomechanical history, may be used either as a shape memory material or a superelastic material. The Ultraflex stent manufactured and sold by Boston Scientific Corporation is an example of a knitted Nitinol stent.
Another type of self-expanding stent that reverts to its original shape because it undergoes only elastic deformation when radially compressed is exemplified in U.S. Pat. No. 1,205,743, issued to Didcott and incorporated herein by reference. Didcott discloses a self-expanding, braided surgical dilator stent particularly adapted for esophageal dilation, but which can be adapted for use in other body vessels. This patent discloses a stent generally in accordance with the stent 10 shown in FIG. 1A hereof. It comprises a hollow tubular member, the wall of which is formed of a series of individual, flexible, thread elements 12 and 14, each of which extends helically around the central longitudinal axis of the stent. A first subset of the flexible thread elements 12 have the same direction of winding and are displaced relative to each other about the cylindrical surface of the stent. They cross a second plurality of helical thread elements 14 which are also displaced relative to each other about the cylindrical surface of the stent, but having the opposite direction of winding. Accordingly, as shown in FIG. 1A, the threads 12 of the first subset cross the threads 14 of the second subset at crossing points 16. FIG. 1A illustrates an embodiment in which the crossing threads are fully interlaced, however, the crossing threads may be interlaced at other intervals, e.g., every other crossing point or every third crossing point.
As the stent is axially stretched, i.e., as the longitudinal ends 18 and 20 are forced away from each other, the diameter reduces, as shown in FIG. 1B. Likewise, if the wall of the stent is constricted so as to reduce the stent""s diameter, the stent elongates. In other words, radial constriction and axial elongation go hand in hand. When the force is released, the stent tends to spring back to its resting diameter and length. The force with which the stent returns to its original state depends on many factors, including the rigidity of the individual threads, the number of threads, and the original (resting) crossing angle, xcex1, of the threads. The rigidity of the threads, in turn, depends upon such factors as the material out of which they are fabricated and the thickness of the threads. In general, the greater the rigidity and/or the greater the resting crossing angle a of the threads, the greater the radial expansion force. The relationships between the stent deformation and mechanical properties as a function of its geometry and material properties is described in Jedwab and Clerc, xe2x80x9cA Study of the Geometrical and Mechanical Properties of a Self-Expanding Stentxe2x80x94Theory and Experimentxe2x80x9d, Journal of Applied Biomaterials, Vol. 4, pp. 77-85 (1993).
The desired radial expansion force for a given stent depends on the application. When used in blood vessels, stents are commonly used to treat stenoses. Accordingly, such applications require relatively high radial expansion forces. Other applications, such as esophageal applications, require much lower forces.
U.S. Pat. No. 4,655,771 issued to Wallsten discloses a stent of the Didcott design particularly adapted for transluminal implantation in blood vessels for treating stenosis and aneurisms.
In some applications, such as the esophageal application particularly discussed in the aforementioned patent to Didcott, the stent is temporary. In other applications, such as the blood vessel application discussed in the aforementioned Wallsten patent, the stent is permanent. In permanent installations, the tissue of the body lumen within which the stent is placed tends to grow around the stent such that the stent essentially becomes incorporated with the tissue of the body vessel and thus becomes permanently affixed. However, in the weeks or months before this occurs, the stent is held in position by friction between the outer surface of the stent body and the inner surface of the vessel created by the radial expansion force of the stent. Thus, the resting diameter of the stent is selected to be slightly larger than the inner diameter of the vessel so that there is a constant force between the inner wall of the vessel and the outer wall of the stent.
Bioabsorbable stents are also known in the prior art. Bioabsorbable stents are manufactured from materials that dissolve over an extended period of time when exposed to body fluids and are absorbed into the surrounding cells of the body.
Various bioabsorbable materials that are suitable for fabricating stents are known in the prior art including polymers such as poly-L,D-lactic acid, poly-L-lactic acid, poly-D-lactic acid, polyglycolic acid, polylactic acid, polycaprolactone, polydioxanone, poly(lactic acid-ethylene oxide) copolymers, or combinations thereof. Vainionp at al., Prog Polym. Sci., vol. 14, pp. 697-716 (1989); U.S. Pat. Nos. 4,700,704, 4,653,497, 4,649,921, 4,599,945, 4,532,928, 4,605,730, 4,441,496, and 4,435,590, all of which are incorporated herein by reference, disclose various compounds from which bioabsorbable stents can be fabricated.
Self-expanding braided stents rely on the spring force of the crossing threads that form the stent body to provide the radial expansion force. The magnitude of the radial expansion force is, therefore, a function of such factors as the number of threads, the size of the individual threads, the moduli of elasticity and rigidity of the thread material, and the initial crossing angle of the threads. Self-expanding knitted stents rely on a separate set of factors, including size and number of threads employed, the flexibility of the individual threads, and the particular knit pattern.
These characteristics of the stent, however, must be chosen based on factors in addition to the desired radial expansion force. For instance, the size of the threads is at least partially limited by the size of the lumen within which it will be employed. Further, characteristics of the material forming the stent body and thus the tensile strength and moduli of elasticity and rigidity of the material is limited to materials which can be safely placed in a human body.
Stents made from bioabsorbable materials exhibit different properties than corresponding metallic stent designs. Examples of properties that must be controlled when using bioabsorbable materials include degradation rates, material creep, and material position memory. These factors also affect the radial expansion force of the stent.
Accordingly, it is desirable to have some other or additional means by which to enhance the radial self expansion force of the stent.
It is an object of the present invention to provide methods and designs to achieve optimum performance employing polymeric materials in stents and more preferably polymeric bioabsorbable materials.
It is another object of the present invention to provide an improved method of attaching to a polymeric stent supplemental elastomeric filaments for increasing the radial expansion force of the stent.
It is further object of the present invention to provide an improved bioabsorbable stent with enhanced radial self expansion force.
It is yet a further object of the present invention to provide an improved bioabsorbable stent with attached supplemental elastomeric filaments for increasing the radial expansion force of the stent.
The invention pertains to methods for attaching to a stent an axial elastomeric filament for enhancing radial expansion force of a self expanding stent, particularly a polymeric, braided, self expanding, stent. It also pertains to stents having attached axial filaments for enhancing the radial expansion force of the stent. In accordance with the invention, the axial filament or filaments are adhesively bonded to the stent body either over their entire length or at intervals. In one embodiment, adhesive is applied with a syringe or a glue gun over the length of the filament. The stent, including the filaments, is then heat treated to cause the glue to bond the filaments to the stent body. Alternately, the glue may be applied only at those points where the axial filament intersects with a thread forming the body of the stent. In another embodiment, the glue is applied to a free standing filament and the filament is subsequently incorporated in or laid on the stent body.
In another embodiment, the entire stent with the axial filament is sprayed with adhesive. In yet a further embodiment of the invention, a portion or all of the stent, including the filaments, is dipped into an adhesive solution.
In another embodiment in which the axial filaments are interwoven with the threads forming the stent body, there is no adhesive and the friction force between the filament and the threads is sufficient to keep the axial filaments in place.
In yet a further embodiment of the invention, a band of adhesive is applied to the stent body with a syringe or glue gun and the adhesive itself forms an axial filament.
In one other embodiment of the invention, the stent is dipped in an elastomer solution. The elastomer is then heat cured. In this embodiment, the elastomer bonds to the threads that form the actual body of the stent. The threads with the elastomer coating are stiffer than the threads without the elastomer coating. Accordingly, the tendency of the stent body to return to the radially expanded position in which it was cured is increased. Any portion of the stent body up to and including the entire stent can be dipped in the elastomer in accordance with this embodiment.
In yet another embodiment, the stent can be held in a radially expanded position when the stent is sprayed or dipped into the elastomer solution and/or during the subsequent heat curing step. After the elastomer is heat cured, the stent will have a larger resting radial diameter than it otherwise would have had.